1. Field of the Invention
The present invention relates to a method to evaluate the R.sub.wb parameter to be used in the interpretation of electrical, acoustic and radioactive oil well logs according to the Dual Water model suggested by C. Clavier et al. in "The Theoretical and Experimental Bases for the Dual Water Model for the Interpretation of Shaly Sands", issued in the SPE Paper 6859 (1977). More specifically, the present invention relates to a process to evaluate the R.sub.wb parameter - resistivity of the water adsorbed to the shales within an oil reservoir rock - through an osmotic filtration process effected on natural clays and/or crushed shales.
2. Prior Art
In the drawing of reservoir logs, constituted by plots of petrophysical measurements against depth, a correlation is sought between changes in a physical parameter and changes in depth, whereby the changes in the parameter allowing the larger or minor presence of oil in said reservoirs to be ascertained. Most of the measurements effected through well geophysical logs are related to the rock porosity, since the higher or lower porosity is directly related to the reservoir oil storage capacity.
Porosity, .phi., the ratio of rock void volume by total volume of the rock, is directly linked to the reservoir water saturation. On its turn, porosity is also influenced by the shaly structure ("shaliness"), or the ratio of shale volume to the pore volume, of a rock V.sub.sh. Generally, porosity is calculated from electric, acoustic and radioactive well logs.
The parameter water saturation, S.sub.w, is expressed by the ratio rock water volume to rock total volume. The amount of hydrocarbons in a reservoir can be determined by difference, if S.sub.w is known, viz, S.sub.hc =1-S.sub.w.
Besides, the concepts of porosity, .phi., and hydrocarbon saturation, S.sub.hc, are directly applied to the evaluation of the hydrocarbon available volume or strategical reserves of a reservoir rock.
The water saturation, S.sub.w, is also related to the resistivity of the rock interstitial water, and to the overall rock resistivity, by the equation of G.E. Archie, as described in the paper "The Electrical Resistivity Log as an Aid in Determining some Reservoir Characteristics" Trans AIME 146, 54-62 (1942): ##EQU1## where a, m and n are empirical parameters, R.sub.w is the resistivity of the rock interstitial water and R.sub.t is the overall rock resistivity, S.sub.w and .phi. being as pointed out before.
H. W. Patnode and M. R. J. Wyllie, in "The Presence of Conductive Solids in Reservoir Rocks as a Factor in Electric Log Interpretation", Trans. AIME 189, 47-52 (1950) recognize the influence of the shaly-structure ("shaliness") on the rock resistivity R.sub.t and the porosity.
In "Electrical Conductivities in Oil-Bearing Shaly Sands", Soc. Pet. Eng. J. 8, 107-122 (1968), M. H. Waxman and L. J. M. Smits established the laboratory basis of the Dual Water concept, for waters contained in oil reservoirs: one kind would be the free water in the pores and the other one would be adsorption-attached She claymineral particles.
This concept has been applied by Ciavier et al. to log data in the above-mentioned publication, so as to introduce the term resistivity of adsorbed water (R.sub.wb), this parameter being derived from the characteristics of resistivity logs related to shales adjacent to the oil reservoirs being examined. However, Waxman and Smits as well as Clavier consider that the resistivity of the shale water situated upwards or downwards the sandstone, R.sub.wb, is the same as that of the shaly fraction within the sandstone. This leads to errors in the measurement of the rock resistivity R.sub.t as well as in the porosity calculated directly from the logs. These errors are equally transferred to the water saturation calculations S.sub.w, as well as to the measurement of the reservoir storage capacity or reserves. Calculations made by Waxman and Smits as well as Clavier will be right only when water concentration upwards or downwards the sandstone be exactly the same as the water of the shaly fraction within the sandstone, which is seldom the case as is also demonstrated in the present invention.
The inaccuracy in the calculations proposed by Waxman and Smits as well as by Clavier et al has been ascertained when petrographers begun the identification, within the pores of the reservoir rocks, of autigenic clays (that is, those generated after the deposition of sandstones through precipitation or alumino silicates directly from the interstitial waters). They realized that the resistivity of the shale waters upwards or downwards the sandstone, R.sub.wb, was different from the water resistivity within the sandstone, R.sub.w. As this error is conveyed to the calculation of the reservoir storage capacity, it is extremely important that true measurements of R.sub.wb be introduced, this being achieved through the osmotic filtration process described and claimed in the present invention.
Therefore, the above-mentioned inaccuracy in calculations comes out from three main and logical reasons:
a) the shales, interlaminated or stratigraphically positioned upwards or downwards the sandstones to be studied by means of the logs, do not necessarily show the same mineralogical characteristics as the autigenic or alogenic clays which are present in the pores of shaly sand rocks, chiefly in the case of the autigenic clays;
b) the shale calculated porosities are inferred from the properties of pure sandstones; therefore, these are apparent and not realistic porosities; and,
c) also, the shales petrophysical parameters, a, m and n are not identical to those of the sandstones.
Therefore, the chief problem in precisely measuring the resistivity of shaly sedimentary rocks comes chiefly from the presence of clayminerals within the pores of the reservoir rocks, where these clayminerals behave as semi-permeable or selfflitrating membranes. This behavior has first been mentioned by L. U. De Sitter in "Diagenesis of Oil Field Brines" Bull AAPG, 31 (11), p. 2030-2040 (1947) and later by F. Bernstein in "Distribution of Water and Electrolytes between Homoionic Clays and Saturating NaCl Solutions" - Proc. 8th. National Clay Conference, Pergamon Press, p. 122-149 (1960). This physicochemical property of shaly membranes is due to a cationic adsorption phenomenon present in the huge clay mineral contact surfaces in order to overcome the negative charge sites, formed by isomorphic substitutions or hydrogen dissociation from structural hydroxyls or even mechanical failure of clayminerals particles. This adsorption gives rise to zones of distinct ionic concentrations adjacent to the claymineral surfaces, while a neutral salinity zone occurs in the more central sites of the porous spaces. Such adsorptive zones are called double electric Zayers. The present invention states that low pressure osmotic filtrations can equally retain or even free salts.
Experimental simulations of incipient, low pressure compactions on artificial shaly muds, suggest that the initial diagenetic processes following sedimentation suffered by sedimentary rocks are of osmotic nature. They have origin different salt concentrations and/or vapor pressure experienced by the solutions within their pores, which entrain variations in the salinity of their filtrates or effluents, this variation being nearly 10%.
Therefore, osmosis is a diagenetic process which is active during the initial compaction stages of shaly muds. The observed variations in the salinities of their filtrates are such that highly concentrated muds show high efficiency in salt retention or filtration, while the more diluted ones free or squeeze out salts. The process of salt retention or filtration in these diagenetic stages is called here osmotic filtration or at low pressure.
There is a point of osmotic stability between the salinities of the mud interstitial solutions and of their effluentes. Below that point the mud retains salt while its effluent becomes more diluted, or sweeter, than the original solution employed to prepare the mud. Above the stability point the mud frees salts and its effluent turns more concentrated, or salty. The stability, or equilibrium point, signals the actual concentration of the water adsorbed to the claymineral particles present in the mud, which can be converted to resistivity, R.sub.wb, and further used in advanced quantitative models of electric, acoustic and radioactive logs, analogous to that proposed by Claylet et al, mentioned before, as well as other similar models.
A main aspect to be considered in these studies is the concept of the clayminerals double electric layer. This double layer is formed by the excess of outward negative charges in the claymineral, the neutralization of which requires positive counter ions, this phenomenon being called Cation Exchange Capacity - CEC. On their turn, counter ions are subjected to two opposite force systems: the electrostatic force which attracts them towards the clayminerals outward contact surface, while a chemical diffusion potential leads them to the inner part of the pores. While in the anhydrous state the counter ions are attached to the clayminerals surface, in aqueous solutions the attractive forces are drastically reduced as a function of the high water dielectric constant, giving rise to an ordered diffusion in the ambient solution, creating zones of ionic concentrations or electric, distinct charges, called double electric layers.
In the diagenetic process which is at the origin of the sedimentary rocks post-deposition modifications, the sediments compaction reaches rather effective high pressures; there is then an overlapping of two electric layers, from neighboring claymineral particles, the consequence being the volumetric loss of ions (Donnan effect), and electric imbalance. On the opposite side of the ionic flow there is an electrostatic cation or salt retention. De Sitter observed this semi-permeability in the above-cited paper. As for the minimum pressure for the shales or clayminerals to behave electrostatically as a semi-permeable membrane, up to now, there is no agreement among various authors, who point out values between 100 and 700 kg/cm.sup.2.
Semi-permeable membranes allow the establishment of an osmotic flow, the consequence of which is the balance of concentrations or pressures on both sides of a membrane. A typical example is the deposition of a shaly mud, which will have initially the same concentration in the solution of its micro and macropores, in spite of different vapor pressures. The lower vapor pressure in the micropores is due to the high cations and water molecules adsorption to the claymineral walls. Through the small separation existant between the macro and micropore, which, according to F. Bernstein in the above-mentioned paper behaves as a semi-permeable membrane, an osmotic flow is established from the macropore to the micropore, the flow ceasing when vapor pressures at both sides of the membrane are balanced. The direction of the osmotic flow causes a higher salt concentration within the macropore and, in case the mud is submitted to pressure or otherwise compressed, the macropore fluids, which are more easily freed, produce effluents which are more concentrated than the original solution. In other situations, the micropore solution is more diluted than that of the macropores, osmosis is established from the micro to the macropore, and the macropore solution turns more diluted. In case this mud is compressed, there are obtained more diluted effluents than the original waters used in the mud preparation. In this latter situation the osmosis effect corresponds to an effective salt retention, very similar to the filtration which would occur in case of an overlapping of electric double layers, or electrostatic filtration.
Therefore, osmotic processes are of utmost importance to determine the salinity of mud effluents or filtrates submitted to pressures such as those occurring in natural diagenetic processes. On the other hand, salinity is linked to the rock resistivity through the equation below: ##EQU2## where F is the formation factor; R.sub.o is the resistivity of a reservoir rock completely saturated (100% saturation) by an aqueous electrolyte of resistivity R.sub.w ;
C.sub.o and C.sub.w are the corresponding conductivities. PA1 a) effect the solids mineralogical identification by means of X-Rays diffratometry; PA1 b) mill the shale cores identified in a), retaining the fraction passing through a 200 mesh Tyler screen; PA1 c) measure the Cation Exchange Capacity (CEC) of step b) solids; PA1 a) prepare brines with varying contents of potassium, magnesium, sodium and calcium ions; PA1 b) measure the resistivity R.sub.w of the original brines prepared in a); PA1 a) prepare muds of 20% solids and 80% liquids concentration, the mud density being 1.4 g/cm.sup.3, the muds being left at rest at least for 72 hours after preparation; PA1 a) Submit the muds prepared in C) to a pressure of 100 psig, (7 kg/cm.sup.2), collect the various filtrates and measure their resistivities R.sub.mf in a precision resistivity meter; PA1 a) Determine the SFE (R) of each mud as a function of resistivity data using the formula: ##EQU4## PA1 a) draw a logarithmic plot of the resistivity R.sub.w of the original brines (item B,b) vs. the filtrates resistivities R.sub.mf (item d), which results in a straight line of correlation coefficient near 1, where the situation of osmotic balance is given by R.sub.w =R.sub.mf. PA1 b) SFE(R) vs. R.sub.w
According to this equation, for a given sample, a plot of C.sub.o vs. C.sub.w should be a straight line of slope 1/F provided that be satisfied the following conditions: a shale-free reservoir completely saturated by water of resistivity R.sub.w. Under these conditions, the formation factor (F) is a rock parameter describing the geometry of its pores. F is independent of C.sub.w such that C.sub.w /C.sub.o vs. C.sub.w for a given sample shall equally give a straight line in a linear plot.
However, for shaly sands and a given value for C.sub.w, the C.sub.w /C.sub.o ratio is reduced, this being attributed to the influence of the rock shale content on C.sub.o. As c.sub.w is reduced, C.sub.o is more rapidly reduced, or, for small values of C.sub.w, there is an extra conductivity, attributed to the shale content. Thus, the electric display of the effects coming from the presence of clay(shale) in the rocks has been described in terms of an "excess conductivity" represented by a factor X in the equation below: ##EQU3##
In log interpretation studies, another important parameter related to the porosity is the water saturation - S.sub.w - which is a function of the porosity and the overall rock resistivity, as well as of the water resistivity, that is, the salinity. G. V. Chilingarian, in "Chemistry of Interstitial Solutions in Shales versus that in Associated Sandstones", SPE paper no. 2527 (1969), provides a thorough discussion on this matter.
The above-mentioned paper by Clavier et al. contains still other important considerations on the correlations existing among porosity-conductivity-water saturation in shaly sands.
The state-of-the-art literature permits to determine a reservoir porosity with the aid of the set of sonic, neutronic and density logs. As there is a direct relationship between porosity and resistivity, more precise resistivity measurements will constitute a precious tool in determining a reservoir porosity, and eventually to better quantify the reservoir hydrocarbon content.